The use of high energy photoionization for the detection of gas chromatograph (GC) eluates was first reported by J. E. Lovelock in Nature, Volume 188, Page 401, 1960. A variety of ionization techniques have been reviewed and compared also by Lovelock in Analytical Chemistry, Volume 33, page 163, 1961. The photoionization detector (PID) was first commercialized by J. N. Driscoll and F. F. Spaziani of HNU Systems, Inc., who have reviewed the development of the PID in Research/Development, Volume 27, Page 50, 1976. Recently, various ionization techniques used in gas chromatograph detectors have been reviewed by P. L. Patterson in the Journal of Chromatographic Science, Volume 24, Page 466, 1986.
A special electrode arrangement for ion detection within a PID is the subject of U.S. Pat. No. 4,013,913, awarded to Driscoll and Spaziani. An electrode arrangement is disclosed in this patent where an anode and cathode are positioned, with respect to the ionizing radiation, such that an annular configuration is defined with the anode directly exposed to the radiant energy and the cathode shielded from the energy source by either a metallic or an organic plastic material. Such an arrangement is said to produce low noise by minimizing the creation of unwanted photoelectrons from UV radiation striking the cathode.
A PID usually includes a radiant energy source (usually high energy UV light of approximately 10 electron volts), an ionization chamber containing ion accelerator and collecting electrodes, and electronic circuitry for driving the photon source, amplifying the ion current and driving an output device. The sample to be detected is passed as a gas through the ionization chamber where it is exposed to the radiant energy and ionized. The ions formed are accelerated and collected by the electrode structures within the ionization chamber.
Early PIDs had the ionization chamber pneumatically interconnected to the light source which consisted of an emission chamber driven by either a d.c. or r.f. discharge. Sealed light sources were later developed which did not have to be operated under vacuum, did not require the use of ultra-high purity gases and were not susceptible to a change in emission characteristics due to contamination of the discharge electrodes. These sources enabled reliable PIDs to be developed and allowed the PID to be commercialized.
There are two primary problems related to present PID designs. First, the UV lamp window is in contact with the sample stream. Although this is desirable to minimize any unswept volume (dead volume), it allows the window to become fogged by nonvolatile components in the sample stream and by polymer products formed when certain GC eluates such as glycols are irradiated by the UV light. Such window coatings reduce the amount of light reaching the sample and cause a corresponding decrease in detector sensitivity. Detector performance will eventually deteriorate below that which is acceptable and then the lamp, which is quite expensive, must be replaced or the window cleaned if possible.
It is important to minimize dead volume since it causes distortion of the gas chromatograph eluate profiles, which decreases the usefulness of the chromatographic system. Consequently, a PID design which prevents the sample stream from contacting the lamp window without the creation of any dead volume would be an obvious advance in PID technology.
The second problem with present PID designs is the limitations that arise when a PID is connected in series with another detector such as an electrolytic conductivity detector (ELCD). In certain analyses it is analytically advantageous to connect the PID and ELCD so that the sample stream first passes through the PID and then through the ELCD. In this manner such sample components as aromatics and halogenated organics can be analyzed simultaneously with the PID and the ELCD, respectively. This enables all of the compounds of interest in such procedures as EPA methods 502.2, 601 and 602 to be analyzed using a single gas chromatographic run. It should be noted that recent EPA method 502.2 actually requires the use of a PID and an ELCD connected in series.
Several present PID designs that are commercially available have adaptations which enable them to be serially interfaced to another detector, however, they are not truly engineered to be interfaced to the ELCD or any other detector. In the past, serially interfacing the PID and ELCD required the detectors to be mounted in their normal respective manners with a transfer tube connected from the PID exit port to the ELCD inlet port. The PID exit port was normally on the outside of the PID detector body and external to the GC oven, whereas the ELCD inlet was within the GC oven. Thus the transfer tube had to be heated and routed back into the oven to be connected to the ELCD inlet. This was usually done by routing the tube through the hole through which the PID detector inlet port protrudes into the GC oven or another drilled hole. This approach works, but suffers from the following limitations: (a) The transfer tube required to interface the PID to the ELCD is a potential source of leaks due to connections and breakage of the transfer tube (if the recommended, but fragile, materials such as fused silica are used); (b) Sensitive materials can be decomposed by any reactive surfaces of the transfer tube; (c) The integrity of the peak elution profiles can be deteriorated by cold spots and unswept volume in the transfer tube and fittings; and (d) Both detector ports of 2-port chromatographs are required for mounting the PID and ELCD detectors, precluding the use of a third detector.